Add log2 support and make log10 obsolete for consistency.

* configure.ac (log2): Check for this function.
* doc/lispref/numbers.texi (Math Functions): Remove obsolete function log10.
* lisp/subr.el (log10): Move here from C code, and declare as obsolete.
All uses of (log10 X) replaced with (log X 10).
* src/floatfns.c (Flog) [HAVE_LOG2]: Use log2 if available and if the
base is 2; this is more accurate.
(Flog10): Move to Lisp (marked obsolete there).

2 files changed, 4 insertions, 5 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index 65fc76d15f5..c00d3392908 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,7 @@
+2013-06-20  Paul Eggert  <eggert@cs.ucla.edu>
+
+        * numbers.texi (Math Functions): Remove obsolete function log10.
+
 2013-06-19  Stefan Monnier  <monnier@iro.umontreal.ca>
 
         * modes.texi (Mode Line Data, Properties in Mode): Advertise `keymap'
diff --git a/doc/lispref/numbers.texi b/doc/lispref/numbers.texi
index eeebac6bf72..2b6f31b670b 100644
--- a/doc/lispref/numbers.texi
+++ b/doc/lispref/numbers.texi
@@ -1156,11 +1156,6 @@ This function returns the logarithm of @var{arg}, with base
 returns a NaN.
 @end defun
 
-@defun log10 arg
-This function returns the logarithm of @var{arg}, with base 10:
-@code{(log10 @var{x})} @equiv{} @code{(log @var{x} 10)}.
-@end defun
-
 @defun expt x y
 This function returns @var{x} raised to power @var{y}.  If both
 arguments are integers and @var{y} is positive, the result is an